The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr), magnetic remanance (Mr), coercivity squareness (S*), medium noise, i.e., signal-to-noise ratio (SNR), and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements.
The linear recording density can be increased by increasing the Hr of the magnetic recording medium. This objective can be accomplished by decreasing the medium noise, as by maintaining very fine magnetically noncoupled grains. Medium noise in thin films is a dominant factor restricting increased recording density of high density magnetic hard disk drives, and is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
A conventional longitudinal recording disk medium on a glass substrate is depicted in FIG. 1 and comprises a substrate 10 of a glass, ceramic, or glass-ceramic material. There are typically sequentially sputter deposited on each side of substrate 10 an adhesion enhancement layer 11, 11', e.g., chromium (Cr) or a Cr alloy, a seed layer 12, 12', such as NiP, an underlayer 13, 13', such as Cr or a Cr alloy, a magnetic layer 14, 14', such as a cobalt (Co)-based alloy, and a protective overcoat 15, 15', such as a carbon-containing overcoat. Typically, although not shown for illustrative convenience, a lubricant topcoat is applied on the protective overcoat 15, 15'.
It is recognized that the magnetic properties, such as Hr, Mr, S* and SNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer which, in turn, is influenced by the underlying layers, such as the underlayer. It is also recognized that underlayers having a fine grain structure are highly desirable, particular for growing fine grains of hexagonal close packed (HCP) Co alloys deposited thereon.
It has been reported that nickel-aluminum (NiAl) films exhibit a grain size which is smaller than similarly deposited Cr films, which are the underlayer of choice in conventional magnetic recording media. Li-Lien Lee et al.,"NiAl Underlayers For CoCrTa Magnetic Thin Films", IEEE Transactions on Magnetics, Vol. 30, No. 6, pp. 3951-3953, 1994. Accordingly, NiAl thin films are potential candidates as underlayers for magnetic recording media for high density longitudinal magnetic recording. However, it was found that the coercivity of a magnetic recording medium comprising an NiAl underlayer is too low for high density recording, e.g. about 2,000 Oersteds (Oe). The use of an NiAl underlayer is also disclosed by C. A. Ross et al.,"The Role Of An NiAl Underlayers In Longitudinal Thin Film Media", J. Appl. Phys. 81(a), P.4369, 1997.
In order to increase Hr, magnetic alloys containing platinum (Pt), such as Co--Cr--Pt-tantalum (Ta) alloys have been employed. Although Pt enhances film Hr, it was found that Pt also increases media noise. Accordingly, it has become increasingly difficult to achieve high areal recording density while simultaneously achieving high SNR and high Hr.
In copending U.S. patent application Ser. No. 08/945,084 filed on Oct. 17, 1997 now U.S. Pat. No. 6,010,795, a magnetic recording medium is disclosed comprising a surface oxidized seed layer, e.g. NiP, and sequentially deposited thereon a Cr-containing sub-underlayer, a NiAl or iron aluminum (FeAl) sub-underlayer, a Cr-containing intermediate layer and a magnetic layer.
There exists a need for high areal density magnetic recording media exhibiting high Hr and high SNR. There also exists a need for magnetic recording media containing a glass or glass ceramic substrate exhibiting high Hr, high SNR, low signal modulation and high off-track capacity.